DESCRIPTION (Applicant's Abstract): Accurate measurement of gastrointestinal absorption of various substances is important in both research and diagnosis. In many cases, the acceptable general approach is the balance procedure. This procedure suffers from the requirement for complete collection of stools over an extended period of time. This limitation prevents widespread use of this otherwise desirable approach. The investigators propose to combine the recent development of Dy as a nonabsorbable fecal marker with the traditional balance approach, using both unlabeled continuous feeding and labeled bolus administration, to develop practical procedures for accurate measurement of gastrointestinal absorption of various substances with a simple "grab sample" approach. During Phase I, the investigators will carry out a study with preterm infants focusing on the development of the approach for accurate measurement of fat absorption. During Phase II, the investigators will carry out in-depth research investigating malabsorption of various substances and/or under different conditions for which there is demand to develop "test kits" for routine use. During Phase III, they will make this new technology available in the form of "test kits" and a central laboratory for complete processing of the kits.